Anticipating his mid-life crisis

Hi Meredith,
I'm a long time reader, occasional commenter. Most of your letters are about breakups, but mine is about preparing for the future. I'll give you some background. My husband and I got married at a young age (early 20s). We are now in our early 30s, have 3 beautiful boys, and a great loving marriage. My husband is a great father and awesome husband, and the list of awesome qualities goes on and on. Although things are fabulous now, I live with one fear every day of my life: I'm scared of his potential mid-life crisis.
I hear from so many women who, like me, were married young, and they tell me how their husbands woke up one morning and decided they wanted something different and had an affair. I'm scared that the same fate lies ahead for me simply because we were so young when we got married and also started a family right away.
So my question for you and your commenters is: What causes a man's mid-life crisis? Is it preventable? What can I do to make certain we grow old together? I'm okay if he buys a sports car or speed boat as a result of a mid-life crisis, but an affair would be devastating! Is my worrying about it like creating a self-fulfilling prophecy?

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Mid-Life Crisis Ahead, South Shore

I bet that if you interviewed these men who got married young and had affairs, MLCA, they'd tell you that they didn't just wake up one random morning with a desire to cheat. I bet they'd tell you that they'd been questioning their marriages for a long time. I have a serious hunch that many people know that their relationships are in trouble by the time they've been married for 10 years.
I'm not telling you that you're 100 percent safe from relationship decay because you've survived a decade – bad things can happen to any good twosome no matter how long they've been together – but the fact that you're happy after 10 years says a lot about your strength as a couple. And know this: A lot of women email me about wanting a fresh start after marrying young. You're just as likely to have a mid-life crisis. And you're pretty confident that you're not going to bail, right? Why would your husband be different?
Don't waste your time worrying about this stuff. Just remind your husband that if there are ever any problems, you want to talk about them immediately.
And maybe when he turns 40, buy him a ridiculous car. Couldn't hurt.
Readers? Should she fear a mid-life crisis? Does marrying young mean that you freak out later? Is there anything she can do to prevent the freakout? Am I right to say that she's just as likely to have a mid-life issue? Discuss.